The present invention relates to chemical-mechanical polishing (CMP) tools, and more particularly, to carriers for holding semiconductor wafers during polishing periods, and specifically to controlling the pressure applied by carriers.
Chemical mechanical polishing (CMP) tools are typically used to planarize the surface of a semiconductor wafer or to remove the upper portion of a layer formed on the semiconductor wafer through any one of a variety of processes, for example damascene processes. Some conventional CMP tools also include a rotating or non-rotating carrier to hold a wafer, and a rotating or orbiting platen or table with a polish pad. The CMP tool causes the polish pad and the wafer surface to come into contact, typically applying a specified pressure between the polish pad and the wafer surface. The CMP tool also imparts a relative motion between the wafer surface and the polish pad. Additionally, the CMP tool typically introduces slurry at the interface between the polish pad and the wafer surface, although some abrasive pads do not require slurry. The slurry can have abrasive particles suspended in a chemical solution that react with selected materials on the wafer surface. The pressure, slurry and relative motion effectuate the polishing.
This planarization or polishing is commonly accomplished by securing the wafer to a carrier, rotating the carrier and placing the rotating wafer in contact with a polishing pad mounted on a platen. A conventional wafer carrier typically includes a hard flat plate that is rigid and so does not conform to the surface of the wafer. The plate surface is therefore covered by a softer carrier film that allows the hard plate to apply a more uniform pressure across the surface of the wafer. This process is known in the industry as back referencing technology. Back referencing has not been entirely successful in that any inconsistencies between the backside of the wafer and the carrier film are translated to the front of the wafer by virtue of the direct contact and the flexibility of the wafer.
In an effort to reduce the amount of non-uniformity caused by the back referencing technology, other systems use an inflatable bladder instead of the soft carrier film. The inflated bladder is apparently intended to absorb the imperfections from the backside of the wafer. This process is known in the industry as xe2x80x9cfront referencing technology.xe2x80x9d
Non-uniform planarization can occur even when uniform pressure is applied to the front surface of the wafer. Non-uniform slurry distribution and the result of different polishing motions applied to different areas of the wafer surface are the most common examples. The non-uniform planarization results are typically manifested as concentric bands on the front surface of the wafer that reflect differences in material removal rate.
What is needed is a carrier in wafer surface topology that reduces the inconsistencies associated with back referencing technology and certain types of front referencing technology.
This summary of the Invention section is intended to introduce the reader to aspects of the invention and is not a complete description of the invention. Particular aspects of the invention are pointed out in other sections herein below and the invention is set forth in the appended claims, which alone demarcate its scope.
In accordance with the aspects of the present invention, a multi-zoned pressure control carrier for use in a CMP tool is provided. This front referencing carrier permits polishing of wafers so the inconsistencies inherent in utilizing multi-zone technology (i.e. the shear force or gradient problems discussed below) can be compensated for and reduced to acceptable levels in addition to allowing for polishing profiles that require multi-zone polishing. Additionally, because the present invention is a xe2x80x9cfront referencing carrierxe2x80x9d, it also provides a solution to back side wafer inconsistencies.
In one embodiment of the present invention, the multi-zoned carrier includes several cells, for example, a center cell, a middle cell and an outer cell. The middle cell might be in fluid communication with the center cell and the outer cell via conduits supplied with flow restrictors, for example, flow orifices. Either the center cell or outer cell is in direct fluid communication with an air supply (or supply of other gaseous fluid), so that one cell receives pressure from the supply while the other cell operates as an outlet. During the polishing process, the volume of air received from the supply, in conjunction with the selected flow restrictor type, determines the pressure in each of the cells, which in turn establishes a polishing profile.
In one example, during operation the center cell receives a volume of fluid and its internal pressure increases to above that of the middle cell to which it is linked by a passage with a conduit and its accompanying flow restrictor. The fluid then attempts to flow to the middle cell through the restrictor-equipped conduit to stabilize the pressure relationship between the two cells. Similarly, once pressure in the middle cell exceeds that of the outer cell, fluid would then attempt to flow into the outer cell, via the restrictor-equipped conduits that interconnect the middle cell and the outer cell until the pressures in all cells are stabilized. The advantage of this apparatus is that if at any time during the process a pressure were to be applied to a cell, for example an inconsistency on the backside surface of a workpiece or wafer, the cell affected could absorb the displacement and due to the properties inherent in fluids, distribute the pressure increase with the interconnected cells. Thus, instead of an inconsistency being forced through a workpiece from the backside surface to the front surface, the inconsistency could be absorbed into the cell due to the fluid displacement allowed by the interconnectivity of the cells.
The cells may each have differing pressures due to polishing profile requirements. For example, one cell might be controlled at a higher pressure, through flow restrictor sizing and fluid flow rates, than another cell. That difference could manifest itself as a pressure gradient between the cells. In this scenario, a middle cell may be interspersed between an inner and outer cell to reduce the pressure gradient between the inner and outer cells.
To obtain a desired polishing profile the interconnections between the cells might include conduits with devices that have controllably variable resistance to fluid flow, for example a needle valve. The appropriate selection of the flow resistance of the conduits allows for maintaining differing pressures between cells, yet still permits fluid communication between the cells. Thus, the middle cell in this embodiment could then be used to reduce the pressure gradient between the center and outer cells and thereby potentially reduce non-uniform planarization results in CMP.
In another embodiment fluid flow is reversed. In this embodiment the outer cell is pressurized from the supply while the center cell operates as an outlet.
Depending on the desired polishing profile, any one of a variety of flow restrictors may be employed to control the rate of fluid flow. Some flow restrictor options include single, porous, and tunable orifices. These various restrictors, coupled with fluid flow rate and pressure selection, allow process engineers to xe2x80x9ctunexe2x80x9d a carrier for a specific polishing profile.